1. Field of the Invention
The invention relates to the articulation of two objects A and B which are movable with respect to one other with two degrees of freedom in rotation, and more particularly to a compact and rapidly-assembled articulation exhibiting great strength.
For the sake of simplicity, reference will be made hereinafter to a system of orthogonal co-ordinates X, Y and Z, the respective roles of the co-ordinates obviously being able to be exchanged without affecting the invention and the sense of the description. Equally, a rotation said to be about X, Y or Z means that this rotation takes place about a geometric axis of rotation parallel to X, Y or Z, without prejudice to the exact position of this geometric axis of rotation.
Jacks, which may be hydraulic, pneumatic or electric, are frequently used in machines for moving two parts of the machine with respect to one another. One part, object A for example, may be the fixed structure of the machine, and the other part, object C for example, may be a movable element of the machine, the jacks constituting object B. In the case where the relative movement of the objects A and C is not pure translation, it is necessary to link the ends of the jack to the objects A and C by articulations exhibiting degrees of freedom in rotation which depend on this movement. Such an arrangement may also be necessary in order to remedy any imprecision in the relative positioning of the objects A and C.
2. Summary of the Prior Art
In the case where one of the objects A or C to be linked pivots with respect to the other about X for example, it is known to connect at least one end of the jack B to the corresponding object A or C by a simple articulation with only one degree of freedom in rotation about X, the said articulation comprising a cylindrical shaft in a bore oriented parallel to X. Because of errors in positioning along X between the objects A and C resulting from manufacturing tolerances, this solution has two drawbacks:
adjustment of the position of the jack along the shaft may be necessary in order to compensate for the errors in positioning resulting from the manufacturing tolerances; and PA1 it may be necessary to have sufficient play between the shaft and the bore to reduce the precision of the adjustment needed, or even to dispense with the adjustment. PA1 the play reduces the precision of the relative movement of the objects A and C caused by the jack; PA1 if it is substantial, the play frees the jack which can turn about itself under the effect of vibration; and PA1 the contact between the shaft and the bore is then maintained at only one end, which increases the contact pressures and can cause degradation of the shaft and of the bore at this end. PA1 a first object A having a shaft defining a first geometric axis, said shaft having a circular cross-section along said geometric axis; PA1 a second object B having a bore surrounding said shaft; and PA1 a bearing having a bearing axis disposed coaxially on said shaft of said first object and within said bore of said second object, said bearing comprising: PA1 bring the inner ring into a position axially aligned with the outer ring and facing the axial aperture, PA1 insert the inner ring into the axial aperture until the male part-spherical bearing surface is inside the female part-spherical bearing surface, this insertion being effected by a simple translation al movement in the X direction, PA1 turn the inner ring by 90.degree. about X, PA1 fit the guide surfaces. PA1 each component can be made from strong materials simply by using conventional machining means, such as turning, milling and grinding, PA1 the bearing can be assembled without crimping or permanent deformation of the components, which allows hard materials to be used, and PA1 more generally, the invention imposes no constraints on the choice of the materials and thus makes it possible, for each of the components, to choose materials which are compatible with each other and with the external components, in terms of frictional characteristics and of resistance to surface pressure.
However, such play in turn leads to several drawbacks:
Because of this, it is known to connect the body of the jack to the fixed structure by means of a swivel, comprising a male part-spherical bearing surface usually attached to the body of the jack, and a female part-spherical bearing surface surrounding the male bearing surface with limited play and usually attached to the fixed structure. However, such an articulation exhibits several drawbacks.
1. The jack can turn about itself, for example under the effect of the vibration of the machine, which can generate stresses in the connections to the jack power source and cause the said connections to be destroyed. Consequently, supplementary anti-rotation means, that is to say means preventing this self rotation, have to be provided.
2. The swivel articulation has to be over-sized in order to withstand the force transmitted by the jack. This is particularly the case when the female part is assembled onto the male part by permanent deformation of the material of which it consists. This material therefore has to remain soft, and the swivel will not be able to withstand substantial forces without degradation of the part-spherical internal surface of the female part, this degradation being caused by the surface pressures which result therefrom. Moreover, this over-sizing has to be increased when the machines exhibit high levels of vibration. The over-sizing of the articulation itself entails an increase in its size and in its mass.
The problem arises particularly in aircraft jet engines in which objects such as the jet pipe flaps, discharge valves or stator vane control rings are actuated by jacks fastened to a fixed structure of large dimensions, the said jet engines being the seat of very high levels of vibration and stresses. These jacks are typically subjected to vibratory accelerations of 12G, i.e. 12 times the earth's gravity. Vital considerations of longevity lead to the articulations being sized to withstand up to 40G.
A swivel bearing is known from U.S. Pat. No. 4,183,590, in which the male part is a sphere comprising two external flats and a cylindrical bore, and the female part is in two parts and comprises a base plate interacting with a U-shaped yoke for entrapping the male part, the wings of the U interacting with the flats so as to guide the male sphere in a plane parallel to the said flats. Such a bearing provides two degrees of freedom in rotation, and is actually capable of withstanding substantial loads, but it exhibits the drawback of being bulky because of the screws necessary for connecting the two female parts together and to the bearing support.